Not true. Pressure builds until it reaches the pressure limit of the radiator cap, THEN it spills over to the expansion tank. Haven't you ever seen a radiator doing an "Old Faithful" or read the caution on a radiator cap?" In days of yore, there was no expansion tank and the coolant spilled onto the street.
PRVs do NOT allow water to back up into the city's mains. Water heater valves do NOT drip or flow at the first increase in pressure. It takes a substantial increase in pressure to trip a water heater relief valve, radiator cap, or PRV. And I do mean substantial.
It's not going to break. Pipes can handle a goodly amount of pressure.
The pressures involved in hot water (or even steam) are much less than the pressures exerted by freezing water. By a couple of orders of magnitude. Steam pipes are just regular pipes and none has ever been known to break from steam pressure. But they will break if exposed to freezing temperatures and not drained beforehand.
Can't help but respond to your comments: Let's take them one at a time:
The car coolant: I said: ****As the coolant temp increases, volume increases, pressure increases until it gets to the radiator cap setting and then the coolant spills over to the expansion tank.*** You said:***Not true. Pressure builds until it reaches the pressure limit of the radiator cap, THEN it spills over to the expansion tank. One of us has a reading comprehension problem--I don't see any basic difference between the two comments.
You said:***PRVs do NOT allow water to back up into the city's mains. If you're a plumber, than get with it because that's not true. PRV's will let allow water back to the main. For your info, below I've pasted a manufactuer's description of his PRV. Note the comment on thermal expansion. So you don't miss it----under certain conditions it allows the expanding water to escape back into the supply main before it can affect the relief valve.-----
Pressures developed by expanding water. A car radiator cap is set for about 15 psi. T&P's I think somewhere between 100-150 psi, steam systems anywhere from 100-200 psi (only a guess). Expanding water in a closed system. Not too hard to come up with a pretty good number. Assume the water temp goes from 50 to 150F. The Specific Gravity (or density) changes about 1.87%. Use the following expression: Delta P=(delta V/V)*BM Delta P= change in pressure Delta V=change in volume V=trapped volume BM= Bulk Modules and for water=312,000psi So: Delta P= .0187*312,000=5834 psi If you think that the pressure required to trip a T&P valve was substantial, what do you think of 5800 psi? How will the typical household components (water heater, washing machine and hoses etc) deal with that level of pressure? That's why you have T&P valves on the water heater and because you don't want water spilling all over the floor all the time that's why expansion tanks are installed so as to absorb the volume changes without the accompanying rise in pressure. MLD
*******The use of a water pressure-reducing valve normally creates a closed system. When water is heated in a closed system, it expands, causing an increase in pressure. This pressure may increase to the set pressure of the relief valve (on the water heater) causing it to drip, thus releasing the expanding water and protecting the system against excessive pressure, This increase in the system pressure over that regulated by the reducing valve is called "thermal expansion pressure".
No. 25AUB by-pass Model water pressure reducing valves, are an economical solution of this annoyance, since under certain conditions it allows the expanding water to escape back into the supply main before it can affect the relief valve.
Effectiveness of the Thermal Expansion by-pass featureis limited to systems where the street main pressure is less than the setting of the heater relief valve. Therefore, the highest allowable pressure setting for the relief valve should be selected for widest effectiveness of a Thermal Expansion by-pass
The public water system pressure in my area is always changing up and down several PSI day and night. I installed an new water heater a couple of years ago and found that an Xtrol type expansion tank for potable water was necessary to keep the water heater's pressure relief valve from popping every so often. I had tried another relief valve first, but the tank was necessary to absorb the pressure spikes. "Funny" thing is my water meter (in my basement) can be observed oscillating forwards and reverse if you look at the spinner dial on it top while you hear the meters chamber measuring the volume flow while this is happening. What is NOT so funny is that the actual dials on the meter seem not to go backwards (the 1/10 cu ft dial) when the spinner is in reverse. I don't think it adds up to any real dollar amount to my bill. But the frequent meter noise is annoying.
Funny how this thread came up as I was considering installing a check valve in the cold water feed to my hot water tank!
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